1. Field of the Invention
This invention relates to high electron mobility transistor (HEMT) devices and method of making the same.
2. Description of the Related Art
GaN based materials have physical and electronic properties that make them attractive for high temperature, high power and high frequency devices. Wide bandgap semiconductors (GaN and SiC) have inherently lower thermal carrier generation rates and higher breakdown fields compared to Si and GaAs, as shown in Table 1 below.
GaN has additional advantages including a high ( greater than 800 cm2/Vs) electron mobility and a high ( greater than 107 cm/sec) electron velocity. Furthermore, high electron mobility transistors (HEMTs) which offer higher mobilities, better charge confinement and higher breakdown voltages can be fabricated in the AlGaN/GaN materials system. Room temperature radio frequency (8-10 GHz) output powers on the order of 6-8 W/mm are theoretically possible in the AlGaN/GaN materials system and power densities as high as 6.8 W/mm have recently been reported (S. T. Sheppard, et al., 56th Device Research Conference, Charlottesville, Va., Jun. 22-24, 1998).
While promising output powers have been reported in AlGaN/GaN HEMTs, materials-related issues continue to limit device performance. Persistent photoconductivity (PPC) and drain I-V collapse have been reported in AlGaN alloys (M. D. McCluskey, N. M. Johnson, C. G Van De Walle, D. P. Bour, M. Kneissl and W. Walukiewicz, Mat. Res. Soc. Symp. Proc. 521 (1998), p. 531) and AlGaN/GaN heterostructures (J. Z. Li, J. Y. Lin, H. X. Jiang, M. A. Khan and Q. Chen, J. Appl. Phys. 82 (1997) 1227). These effects arise from carrier trapping and generation from deep levels in the material and can lead to poor high frequency performance, decreased drain currents and reduced output powers in a HEMT. PPC and current collapse in GaAs-based HEMTs have been attributed to defect-donor complexes (DX centers) in AlxGa1xe2x88x92xAs when x greater than 0.20. Evidence for oxygen DX-centers in Al-rich AlxGa1xe2x88x92xN (x greater than 0.27) has recently been reported (M. D. McCluskey, et al., ibid.). High Al content AlGaN layers (x greater than 0.20) are commonly used to achieve high sheet densities in AlGaN/GaN HEMT structures via piezoelectric-induced doping as shown by the data in FIG. 1, which is a plot of sheet density as a function of percent aluminum composition in undoped 23 nanometer AlGaN/GaN heterostructures.
In order to further improve the performance of III-V nitride HEMTs, methods must be identified to reduce or eliminate the deleterious effects of deep level defects that result from the use of high Al composition layers.
The present invention relates in one aspect thereof to a gallium nitride-based HEMT device, comprising a channel layer formed of an InGaN alloy.
Such device may comprise an AlGaN/InGaN heterostructure, e.g., in a structure including a GaN layer, an InGaN layer over the GaN layer, and an AlGaN layer over the InGaN layer. The AlGaN layer may be doped or undoped, as necessary or desired in a given end use application of the HEMT.
Alternatively, the HEMT device of the invention may be fabricated as a device which does not comprise any aluminum-containing layer, e.g., a GaN/InGaN HEMT device or an InGaN/InGaN HEMT device.
In another aspect, the invention relates to a method of fabricating a GaN-based HEMT device, comprising forming a channel layer for the device of an InGaN alloy.
Other aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.